Basis is a multiplier used to calculate the total interest paid or received in a defined time period. It is typically used in accounting and valuation for bonds, swaps, dividends, project finance, and other investments with periodic cash flows. Basis is also known as the "day count convention" because it defines how days in a year are treated for calculation purposes in different parts of the world.

30/360
Number of days in the interest period on the basis of a year of 360 days with 12 30-day months, divided by 360. For instance, if the interest period is from May 18 to Aug 18, the number of days in the interest period is 3x30=90. If the end of the interest period is the 31st of a month, but the beginning is a day other than the 30th or 31st of the month, then the last month will not be shortened to a 30-day month. Where the period length is one month and three days, the one month is assumed to have 30 days (irrespective of whether it has more), plus the three days resulting in a period of 33 days.

30/360E
Number of days calculated on the basis of 12 months of 30 days each, over 360 days. The 31st day of a month is skipped.